This invention relates generally to ultrasonic diagnostic systems that use ultrasonic transducers to provide diagnostic information concerning the interior of the body through ultrasonic imaging, and more particularly, to an apparatus and method of forming electrical connections to transducers used in such systems.
Ultrasonic diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements. For example, cardiologists, radiologists, and obstetricians use ultrasonic diagnostic imaging systems to examine the heart, various abdominal organs, or a developing fetus, respectively. In general, imaging information is obtained by these systems by placing a scanhead against the skin of a patient, and actuating an ultrasonic transducer located within the scanhead to transmit ultrasonic energy through the skin and into the body of the patient. In response to the transmission of ultrasonic energy into the body, ultrasonic echoes emanate from the interior structure of the body. The returning acoustic echoes are converted into electrical signals by the transducer in the scanhead, which are transferred to the diagnostic system by a cable coupling the diagnostic system to the scanhead.
The acoustic transducer is a piezoelectric element that is generally made of a crystalline material such as barium titanate, or lead zirconate titanate (PZT). The transducer may be a single element, or it may consist of a single piece of piezoelectric material that is cut, or diced into an array of fine elements, with the individual elements of the array transducer generally being rectangular in shape. Array scanheads are operable in ultrasonic scanning modes known as linear array and phased array modes, in which groups of elements are actuated and used for reception in various combinations. When the elements are used to transmit and receive ultrasonic energy at frequencies in excess of 7 MHz, the physical dimensions of the individual elements can be quite small, with width ranging down to only a few thousandths of an inch. Additionally, the numbers of these finely dimensioned elements can be quite large, with numbers ranging, for example, from 128 elements to in excess of 380 elements. When the number of such piezoelectric elements is large and the physical dimension of the individual elements is small, considerable difficulty is encountered in accurately and reliably making the necessary electrical connections to the individual elements of the acoustic transducer.
According to one prior art method, an electrical lead is soldered to each piezoelectric element in an array by first preparing a surface area on the element to receive the lead. This is generally accomplished by depositing a thin layer of gold on a contact area of the piezoelectric element. The electrical lead is generally comprised of a thin copper strip that has been locally electroplated with a low melting point metal, such as indium, in a contact area of the lead. A flux compound is generally applied to either or both of the surfaces to be joined before the soldering operation. The flux material is required to substantially dissolve a thin film of oxides, or other contaminants that may exist on either of the metallic surfaces, or on the surface of the solder, that may interfere with the formation of metallic continuity between the gold surface on the element and the indium surface on the lead. The lead contact area is then positioned onto the gold contact area of the element, and soldered to the element by a thermal conduction method that generally interposes a eutectic solder alloy between the gold and indium metal layers.
A significant drawback present in the foregoing prior art method is the application of a high temperature heat conduction element to the connection to rapidly fuse the solder alloy to form the metallic connection. Since most transducer materials exhibit a sensitivity to elevated temperatures that potentially renders them vulnerable to damage at ordinary soldering temperatures, pulse reflow bonding machines have been widely used in the manufacture of transducer arrays. In pulse reflow bonding, a resistance thermode applies a pressing force to the connection and then rapidly raises the connection to the solder fusion temperature to form the required connection. The successful application of pulse reflow bonding to transducer manufacture requires, however, precise and uniform temperature control, as well as precise control of force applied to the thermode. Accordingly, pulse reflow bonding equipment constitutes a significant capital expenditure, thus increasing the cost of the completed assembly. Additionally, such equipment tends to be large, thus occupying a significant portion of the plant floor area.
An additional drawback present in the foregoing prior art method involves the post-soldering step of washing the remaining flux and various contaminants from the soldered connection. Since most commercially available fluxes are generally comprised of organic or inorganic acids or halogens, undesired concentrations of ionic compounds may remain on the soldered connection after the soldering process has been completed, which may eventually lead to corrosive damage of the connection. As a consequence, the soldered connection is usually subjected to a washing procedure to remove a substantial portion of these ionic contaminants. In washing the connection, water may be used, or various other organic solvents may be employed. As a consequence, the transducer array must be allowed to air-dry, or alternatively, be placed in a drying chamber before further processing of the element array takes place, thus incurring manufacturing delays. Further, the water used in the washing process may contain the various ionic contaminants, thus necessitating the removal of these contaminants from the water prior to disposal of the washing water into the wastewater disposal system.
Another problem associated with the prior art soldering method is ensuring that the gold surface areas remain free of various contaminants prior to the soldering operation. For example, prior to the soldering step, other structures, such as impedance-matching devices are added to the transducer array and they are attached using a variety of well-known adhesive compounds. Since the structures may be located near the gold surface areas on the elements, meticulous care must be taken to avoid the inadvertent spreading of the adhesive onto the gold surface areas during the joining process. If the adhesive spreads onto the gold surface areas, it must be removed, generally by mechanical means, which is followed by washing the affected area with an organic solvent, thus introducing undesired and meticulous rework of the array, before the soldering step occurs. Moreover, the additional handling incurred during rework operations of this kind may significantly enhance the likelihood of imparting physical damage to the array.
Other prior art methods have supplanted the soldering process described above with a variety of conductive adhesives. For example, U.S. Pat. No. 4,404,489 to Larson, et al. describes the use of a conductive epoxy to attach the electrical leads to the piezoelectric elements. Although this method avoids the use of thermal soldering processes, considerable care must still be exercised in the application of the epoxy during the assembly procedure since the conductive epoxy may form undesired conductive paths to adjacent elements or to grounded structures unless carefully applied.
Still other prior art method employ anisotropic, thermosetting conductive adhesives that contain small, conductive particles that, when compressed and subjected to heat, bond the electrical lead to the conductive pad. Although this method utilizes temperatures that are generally in the range of 80-100 deg. C., which are significantly below typical solder fusion temperatures, pulse reflow bonding equipment is generally utilized in order to apply the required heat and force to the bond, which necessitates a significant capital expenditure, as described above. In addition, the use of anisotropic conductive adhesives generally requires the application of somewhat higher forces to achieve acceptable bonding between the lead and the conductive pad, which generally requires that the application of force must be more carefully controlled, in order to avoid exceeding prescribed material limits.
The invention is directed towards an apparatus and method for forming electrical connections in an acoustic transducer wherein a non-conductive bonding material is interposed between a conductive surface on the transducer and a conductive lead that is coupled to a device that is capable of receiving and transmitting ultrasonic signals. In one aspect, the conductive surface is comprised of gold, and the conductive lead is comprised of copper that is plated with at least one metallic layer, that may be further comprised of an intermediate metal layer that is overlaid by a layer of gold. The intermediate layer may be further comprised of titanium, or alternatively, the intermediate layer may be comprised essentially of an alloy of nickel and chromium. A non-conductive bonding material is deposited on either the metallic layer on the lead or the conductive surface, which are joined to form a bonding interface. Electrical conduction at the interface is attained through a plurality of interfacial contact points that arise from the surface roughness inherent in the materials that project through the bonding interface and establish metallic contact. Alternatively, the interfacial contact points are impressed in the surfaces to augment the metallic contact. In another aspect, the electrical connections and the impedance matching layers are bonded to the piezoelectric material with the non-conductive bonding material at the same processing step to form a transducer array.